The present invention relates generally to a hinge for a door and more particularly to an earthquake-proof hinge which can positively permit the door to be opened even when the gate frame of the door is distorted and interferes with the door due to an earthquake or the like.
When a gate frame is distorted due to the impact of an earthquake or gas explosion, the gate frame interferes with the door so that the door cannot be opened. Needless to say, a very dangerous condition exists when a fire occurs and a door is kept closed.
The reason why the door cannot be opened when the gate frame is distorted will be described with reference to FIGS. 1, 2 and 3. It is assumed that forces are exerted in a plane containing a gate frame 1 so that the gate frame 1 is distorted into the form of a parallelogram. When the lower side of the gate frame 1 is assumed to be held stationary, the forces exerted on the gate frame 1 may be divided into a force .sym. which is directed from the knob 2 of the door 4 to the hinges 3 and a force .crclbar. which is directed from the hinges 3 to the knob 2. When the force is exerted on the gate frame in the direction .sym., the gate frame 1 is distorted as shown in FIG. 2. That is, the upper side edge of the door 4 strongly interferes with the upper side of the gate frame 1. As a result, the door 4 cannot be opened due to the distortions of the door 4 and the gate frame 1 and to the frictional forces therebetween. On the other hand, when the force or impact is exerted in the direction .crclbar., the gate frame 1 is distorted as shown in FIG. 3. That is, the lower side edge of the door 4 strongly interferes with the lower side of the gate frame 1.
In general, the clearance or spacing .beta. between the lower side edge of the door 4 and the gate frame is, for instance, about six millimeters and is greater than the clearance or spacing .alpha. between the gate frame 1 and the upper side edge of the door 4 which is, for instance, about three millimeters. As a result, the tolerance .gamma. in the horizontal direction (See FIG. 2) allowed when the force is applied in the direction .sym. is less than the tolerance .delta. (See FIG. 3) allowed when the force is applied in the direction .crclbar..
Referring to FIG. 4, the conventional so-called "flag" hinge has a first metal plate 5 which is attached to a gate frame 1 and has one vertical side edge formed integral with a pin 6. The hinge also comprises a second metal plate 7 which is attached to a door and has one vertical side edge formed integral with a knuckle cylinder 8 into which is rotatably fitted the pin 6. The load of the door which is axially applied to the pin 6 is carried by the engagement of the upper end of the pin 6 with the top of the knuckle cylinder 8. As a result, the door 4 can be moved upwardly relative to the gate frame.
It follows therefore that when a force is applied to the frame 1 in the direction .sym., the horizontal tolerance .delta. is increased so that when the force is applied in the direction .sym., the door can be opened even when the gate frame 1 is distorted. However, as shown in FIG. 2, when the force is applied in the direction .crclbar., the door 4 cannot be opened even when the gate frame 1 is only slightly distorted. This is very dangerous as described before.
In order to overcome the above described problem, the doors are so designed and constructed that there may be a clearance or spacing of the order of 10 mm between the door and the gate frame. But such countermeasure cannot be applied to existing doors. Furthermore, the outer appearance is degraded and air-tightness cannot be maintained.